The networking of control units, sensors and actuators using a communications system or a bus system has increased dramatically in recent years not only in modern motor vehicle manufacturing and in engineering, especially in the machine tool sector, and automation technology and other industrial applications, but also in the private sector, for example in bus systems for domestic buildings. It is possible in these cases to obtain synergetic effects by distributing functions among several control units. The term distributed systems is used for this. Communication between various stations of such a system is increasingly taking place via at least one bus or at least one bus system. The communications traffic on the bus system, access and receiving mechanisms, and error handling are governed by a protocol.
A protocol that is established in the automotive sector and which is also being used to an increasingly greater extent in other applications is CAN (Controller Area Network). This is an event-triggered protocol, that is to say, protocol activities such as transmission of a message are initiated by events that originate outside the communications system. Unique access to the communications system or bus system is resolved by priority-based bit arbitration. A pre-requisite for this is that each message be assigned a priority. The CAN protocol is very flexible; it is therefore possible for further nodes and messages to be added without any difficulty as long as there are still free priorities (message identifiers) available. The collection of all of the messages to be transmitted in the network, including priorities and their transmitting nodes, and possibly receiving nodes, are stored in a list known as the communication matrix.
An alternative approach to event-triggered, spontaneous communication is the purely time-triggered approach. All communication activities on the bus are in that case strictly periodic. Protocol activities such as the transmission of a message are triggered only by the passage of a time applicable to the entire bus system. Access to the medium is based on the allocation of time ranges in which a transmitting station has an exclusive transmission right. The protocol is comparatively inflexible, and adding new nodes is possible only if the corresponding time ranges were left free beforehand. This circumstance forces the order of the messages to be set before operation is started. At the same time, the positioning of the messages within the transmission periods must also be synchronized with the applications producing the contents of the messages so that the latencies between the application and the instant of transmission are kept to a minimum; otherwise, that is to say, if that synchronization is not performed, the advantage of time-triggered transmission—minimal latency jitters when the message is being sent over the bus—would be destroyed.
The approach using time-triggered CAN, the so-called TTCAN (Time Triggered Controller Area Network), which is described in German Patent Application Nos. 100 00 302, 100 00 303, 100 00.304 and 100 00 305, and in ISO Standard 11898-4 satisfies the requirements outlined above for time-triggered communication and satisfies the requirements for a certain degree of flexibility. The TTCAN fulfills those requirements by structuring the communication round (basic cycle) into so-called exclusive time windows for periodic messages of specific communications stations, and into so-called arbitrating time windows for spontaneous messages of a plurality of communications stations. The TTCAN is generally based on time-triggered, periodic communication which is clocked by a station or node giving the main time, the so-called time master or timer, using a time reference message or short reference message. The period to the next reference message is referred to as the basic cycle and is subdivided into a specifiable number of time windows. A distinction is made between the local times, or the local timers, of the individual stations and the time of the timer giving the global time. Further fundamental principles and definitions relating to the TTCAN will be explained hereinafter or may be learned from ISO 11898-4 and the related art described above.
In the case of TTCAN bus communication, communication objects, especially messages, that are defective and that are marked and made invalid by an error frame are not repeated so as to avoid any risk of exceeding the time window or the cycle time by repeating the message and thereby impeding the message that follows. The receiving communication object for that destroyed message continues not to be updated until a message is received without error in an associated time window. In contrast to this, a defective reference message identified by an error frame is repeated, since it is not possible to do without that reference message. That repetition of the message results in the basic cycle affected being extended by the time from the beginning of the first defective reference message to the beginning of the reference message transmitted without error. Each of those errors leads to a further delay in the timing, with the result that those delays add up to a greater and greater deviation from the nominal time. Such a fault, for example a reference message transmitted with errors, accordingly leads to a time change or deviation from the nominal settings in the system. If two or more TTCAN buses or bus systems are in synchronized operation, such a time deviation, especially a delay, on one of the bus systems must be put into effect on the other bus system in order to obtain synchronism again. Accordingly, such time deviations on all the bus systems are added to one another and the fault or error is propagated.